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All-Optical Photomechanical Devices

All-Optical/Mechanical Circuit

A significant step in developing an all optical technology was reported in 1994 by Kuzyk and Welker, who demonstrated all 5 all-optical device classes in an all-optical position stabilizer.[1] The position stabilizer, diagramed below, keeps the position of the hanging mirror fixed, and operates as follows.

Light from a laser is launched into an interferometer (dashed box). One of the mirrors of the interferometer hangs from a photomechanical optical fiber, whose length change is proportional to the light intensity. The output of the interferometer is launched into the hanging fiber, thus providing optical feedback, as follows.

The intensity of light leaving the interferometer depends on the length of the fiber. So, if the length of the fiber changes due to an external agent, so does the intensity leaving the interferometer. This intensity change causes the length of the fiber to activity change to oppose the external agent, thus preventing the hanging mirror from moving.

Welker and
Kuzyk found that a 30cm fiber could be actively kept constant to within about 3 nm, or one part in 108. The fiber also acted as a positioner - the length of the fiber could be changed by varying the laser power; and, as a digital positioner - by applying mechanical shocks to the system, it would jump through several discrete length states, thus acting mechanically as does a digital electronic device.

To summarize, in this device (1) information about the fiber length is transmitted on a beam of light (optical transmission); (2) the fiber length changes length in response to light (optical actuation); (3) the light beam senses the position of the mirror (optical sensor); (4) the interferometer determines the required change in intensity to resist external stress (optical logic); and (5) the whole optical circuit is powered by a laser. Thus, this device demonstrates that an all-optical device technology is possible.

Photomechanical Optical Devices (PODs)

A breakthrough was made in 1995 when the all-optical/mechanical device was miniaturized into a sub-milimeter device[2] whose operation is identical to the all-optical/mechanical circuit. The device, made from a short segment of a polymer optical fiber is diagrammed below.

One method for making mirrors are to burn Bragg gratings onto the two ends of the fiber (shown as stripes) using a high powered laser. A bragg grating is a series of parallel planes of elevated or depressed refractive index. Each of these planes reflects a small portion of the light, but when many such planes are taken together, the reflectivity can approach 100%.

An important property of a Bragg grating is that it reflects only the color of light whose wavelength is properly matched to the period of the grating. The interferometer formed by the two reflectors controls the light's intensity within. Since the material is photomechanical, the logic, sensing and actuation is all contained within this basic element, called a Photomechanical Optical Device (POD).

A POD has two inputs and two outputs. The light power that exits a POD depends on the input power, the externally applied stress, and its mechanical and optical history (two POD's that have been exposed to different light levels and stresses will behave differently). Thus, we can think of a POD as a neuron in the brain with the added features that it acts as a sensor and is an actuator.